Dec. 5, 1961
THECARBON SKELETONOF SARPAGINE
[CONTRIBUTION FROM THE DEPARTMENT O F CHEMISTRY,
4801
MASSACHUSETTS INSTITUTE OF TECIXNOLOGY, CAMBRIDGE 39,MASS.]
Application of Mass Spectrometry to Structure Problems. of Sarpagine2
1V.l The Carbon Skeleton
BY K. BIEMANN RECEIVED JUNE 27, 1961
It is demonstrated that the mass spectra of indole alkaloids are characteristic of the particular alicyclic carbon skeleton present and that this fact can be used t o compare such compounds even if they differ in the substituents attached t o t h e aromatic part and are, therefore, not suitable for a direct comparison by the use of physical constants. This is demonstrated by the correlation of the carbon skeleton of sarpagine with a derivative of ajmaline. The spectra of I V (from ajmaline) and V (from sarpagine) are almost identical with the exception of a shift of 16 mass units because the over-all difference in their substituents is one oxygen atom. The two functional groups present in the alicyclic system of sarpagine were replaced by deuterium atoms and their positions located on the basis of mass spectra. A detailed discussion of the spectra is presented.
I n the elucidation of the structure of an alkaloid, the presence of an aromatic system-its type and frequently also the position of substituents-is commonly deduced from its ultraviolet spectrum, or from the products formed in drastic degradations. The aliphatic part of the molecule is in general more reluctant to a speedy elucidation, and lengthy chemical degradation or conversion to a known compound is necessary. This approach not only is often very time-consuming but also requires a considerable amount of material, which is not always available. While infrared spectra provide an indication of the presence or absence of functional groups, conclusive information regarding the carbon skeleton cannot be obtained in this way. Mass spectrometry, on the other hand, promises to be a valuable method for the characterization and identification of the aliphatic or alicyclic part of such molecules. While in simpler cases the mass spectrum can be unequivocally interpreted in terms of the structure of the side chain, this is a t present not possible with the high confidence required for structure determination if the molecule in question is a rather complex one. Model compounds considerably aid in such situations and the comparison of two possibly related compounds is a special case of this type. The following investigation will illustrate this point. A molecule consisting of an aromatic and an aliphatic or alicyclic part will on electron impact3 undergo fragmentation mainly a t bonds which are part of the saturated moiety of the molecule, while the aromatic part is much more resistant to fragmentation. The probability of cleavage of a given bond is dependent upon the bond energy, the stability of the positively charged fragment and also of the neutral one formed. The fragmentation pattern oi two indole alkaloids, for example, whichexcept for differences in small substituents in the aromatic part-have the same alicyclic system, (1) Part 111, K Biemann, C Lioret, J. Asselineau, E. Lederer and J Polonsky,Bio'hcm. Biophys A c f o , 40, 369 (1960) (2) Presented a t the International Symposium on the Chemistry of Natural Products, Melbourne, Australia, August, 19GO Part of this work was the subject of a preliminary Communication: K Biemann. Tetrahedron Letters, 16, 9 (1960). (3) It should be recalled that in the rather complex over-all process, an electron is first removed from the molecule, M, to form a positive molecular ion M+. The latter may decompose into a positive and a neutral fragment, a step which can also occiir repeatedly leading to a stepwise multi-fragmentation.
-e-
M.
--t
M + -+ Ff
+ FO (radical or molecule)
will be very similar with the exception of certain constant differences in the mass of the fragments because the above-mentioned factors governing the fragmentations are almost the same in both molecules. As only the positively charged fragments appear as peaks in the spectra, we will in general find two groups of peaks. Those fragments retaining the positive charge on a part of the molecule not bearing the groups responsible for the difference between the two compounds will give rist to peaks of the same mass in both spectra. Another set of peaks, due to positively charged fragments retaining these groups, will appear a t masses shifted in one compound for the net difference in the mass of the additional substituents. Before discussing the application of this principle on a specific example, i t should be pointed out that a close similarity of the spectra with respect to intensity ratios can be expected only if the change in substitution of the aromatic ring does not appreciably influence the stabilization of the positive charge in certain fragments. An increase in stabilization would, of course, increase the intensity of the corresponding peak and tiire versa. This effect is, however, not pronounced in the case of indole derivakives in which these additional substituents, if strongly electron-donating or electron-withdrawing, are in the benzene ring, while the alicyclic system is attached to the pyrrole ring. A methyl group on the indole nitrogen is also not expected to alter significantly the mode oi fragmentation of a xrolecule containing a number of bonds more easily cleaved because the loss of this methyl group would lead t o a rather unfavorable ion with an electron deficient pyrrole nitrogen. The successful application of the principle stated above is illustrated in the correlation of the alicyclic carbon skeleton of sarpagine with theone of ajmaline. In 1953, Stoll and Hofmann4 reported the isolation of sarpagine from RauwolJia serpentina Benth. and the same alkaloid has been found also in other Rauwolfia species. A few years later several g r o ~ p s simultaneously ~-~ suggested structure Ia for (4) A. Stall and A. Hofmann, Hela. Chim. Acta, 36, 1143 (1953). ( 5 ) D. Stauffacher, A. Hofmann and E. Seebeck, Hclv. C h i m . Acta, 40, 508 (1957). (6) J. Poisson, J, LeMen and M.-M. Janot, B u l l . SOL.Chim. Fuance, 610 (19.57). ( 7 ) S . K. Talapatra and A. Chatterjee, Scicficc and Cullure (Calculta), 2 2 , G92 (1957). (8) W . Arnold, W. von Philipshorn, H. Schmid and P. Karrer, Helv. Chim. A d a , 40, 705 (1957).
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net mass difference between the substituents on the indole nucleus (methoxyl in V vs. methyl in IV) is due to one oxygen, ;.e., 16 mass units. The peaks in 2'2 295 the mass spectrum of V should thus be found 16 mass units higher than in the spectrum of IY. 220 240 260 280 300 Compound V was prepared from sarpagine by 0methylation, tosylation of the alcoholic hydroxyl and reduction with lithium aluminum hydride to phenol-0-methyl-deoxysarpagine, which had been '*/e 20 IS0 160 I80 200 220 240 260 280 prepared previously from C-Alkaloid T lochnerine. Fig. 1.-Mass spectra of compound V ( a ) derived from 0-methylsarpagine (Ib).6 For the methylation of sarpagine and of compound I V ( b ) derived from ajmaline. sarpagine we found trimethylanilinium hydroxide6 to be superior to dimethyl sulfate.6 The O-methylA11 peaks 3 2% of the intensity of the peak of the molecular ion are shown in the mass region indicated on the abscissa. sarpagine obtained in this experiment, 29.3 milligrams, was tosylated and half of the crude product The peak a t m/e 182 in ( b ) is 1.45 times as intense as the (IC)reduced to O-methyldeoxysarpagine.8 Hydropeak at m/e 294. genation of 2 milligrams of this product yielded 0sarpagine on the basis of ultraviolet and infrared methyldeoxydihydrosarpagine (V), m.p. 237-240 '. data which revealed a 5-hydroxyindole moiety unThe mass spectrum of this compound is shown in substituted on nitrogen and a free alcoholic hy- Fig. 1 along with the spectrum of compound IV. droxyl group. Chemical reactions suggested the Both spectra are, in fact, very similar but exhibit a presence of a tertiary nitrogen, a C-methyl and a shift of 16 mass units. On the basis of the argu>C=CH-CHZ group and that the alcoholic hy- ments advanced earlier in this paper, we conclude droxyl is very probably -CH20H attached to a that compound IV and O-methyldeoxydihydrohighly substituted carboF atom. The alicyclic car- sarpagine must have the same carbon skeleton atbon skeleton in structure Ia was, however, based tached to the CY- and @-positionsof the indole nucleus and the latter of the two substances is therefore correctly represented by structure lr (stereochemistry a t and Czo unspecified; see discussion below). 9 number of points deserve further comment here. The many additional small peaks in Fig. l a Ia, R = R ' = H compared with lb are due to more pronounced b, R = C H , , R ' = H contributions of the background (traces oi grease?) C, R=CHB, R ' = T s because of the lower volatility of the sarpagine derivative leading to a lower absolute intensity of the spectrum. The peaks a t mass 279 and 250 in Fig. l a , whose counterparts a t mass 263 and 264 are absent in Fig. Ib, are due to loss of the methH k J oxy1 group (masss 31) as such and as CH?O I1 (mass 30). The doubly charged molecular ions (AI++) differ, of course, by S mass units. While the spectra were obtained from mass 24 to mass 400, only the part from about mass 140 to the molecular weight is shown in Fig. 1. The peaks a t lower masses are of relatively low intensity and not charentirely on biogenetic considerations and only the acteristic, The location of the two functional groups which French group had isolated in a selenium dehydrogenation a trace of a product,6 the ultraviolet spec- were removed in the preparation of 7\ remained now trum of which suggested the chromophore 11. to be determined. For this purpose another part of The close relationship of structure Ia to ajnialine the tosylate of 0-methylsarpagine was treated with (III)gwas generally recognized, but none of the in- lithium aluminum deuteride to mark the original vestigators reported a successful interconversion of position of the hydroxyl group, followed by hydrosarpagine into a derivative of ajmaline, which would genation of the double bond as before. The mass have shed some light on the soundness of the struc- spectrum of monodeuterio-I' is shown in Fig 2b. The molecular weight of 311 indicates that one atom tural proposal discussed above. Such a correlation was, however, expected to be a of deuterium was incorporated, as expected. Part simple task employing the mass spectrometric of the peak originally a t mass 295 due to the loss of a principle outlined earlier in this paper. The avail- methyl group has now shifted to 296. This finding ability of a degradation product (IVjln of ajmaline implies that methyl groups are lost as CH3or CHLD required, for such a comparison, O-methyldeoxy- and that the hydroxyl was therefore located on a dihydrosarpagine(V) because both these com- terminal CH3-group. I t is not, however, part of Clg) because the latter is pounds should have the same alicyclic carbon skele- the two carbon chain (cI& ton if structure Ia for sarpagine were correct. The still lost as 29 mass units giving rise to a peak a t mass 282 instead of 281 in the non-deuterated (9) R. B. Woodward, A n g e v Chem., 68, 13 (1956). molecule. The position of the hydroxyl group on (10) Obtained from deoxyajmalalo b y Wolff-Kishner reduction: is thus corroborated. R.B. Woodward and K. Schenker, unpublished. I
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253
Dec. 5, 1961
THECARBON SKELETON OF SARPAGINE
The double bond was located by a similar deuteration experiment. O-Methyldeoxysarpagine was catalytically deuterated using D2-Pt-CH30D. The product was then treated with HZ-Pt-CH30H to exchange the labile deuterium atoms a t the indole nitrogen and possibly in the aromatic system. The mass spectrum of “dideuteri0”-O-methyldeoxysarpagine (Fig. 2c) indicates the presence of up to six deuterium atoms, two of which were introduced by addition of deuterium t o the double bond and the other four by exchange on the neighboring saturated carbon atoms. All, but one, deuterium atoms are located in the ethyl group because the fragment due to loss of that group is now found a t mass 282. The series of peaks beyond 282 are due to the fragments formed by loss of the methoxyl group (mass 279 and 280 in V), therefore containing all the deuterium atoms. While Chis finding permits us only to place the double bond in sarpagine either a t CHCzo,or C18-ClS,the former position is preferred because the incorporation of one full atom of deuterium a t C20 (retained in the fragment of mass 282) is more probable by addition rather than exchange. I n view of the fact that the exchange of hydrogen for deuterium on saturated carbon atoms is very facile in aliphatic molecules, as dramatically shown by the incorporation of up to 18 deuterium atoms into methyl stearate during catalytic deuteration of methyl oleate,” our results indicate that the steric arrangement around the carbon-carbon double bond has considerable influence on the extent of such an exchange. Catalytic deuteration of the double bond may thus serve as a tool for the elucidation of its environment. The mass spectra discussed in the preceding section represent conclusive evidence for structure Ia for sarpagine with the exception of the stereochemistry a t C16and the cis or trans relationship of Cls and C21. Mass spectra are, in general, rather insensitive toward such differences except if they would lead to differences in the stability of the molecular ion or of certain fragment ions as will be shown in a forthcoming paper on the boga alkaloids. Though this is a certain disadvantage of such mass spectrometric correlations, it may frequently turn into an advantage because compounds of different stereochemistry but otherwise identical structure may be compared in this way. It would not be possible, however, by other means like melting points or infrared spectra. While the arguments advanced for the structure of sarpagine (Ia) are of a novel type but nevertheless compelling, i t is gratifying to note that almost simultaneously with our preliminary communication on this subject2 there appeared another onelZ reporting the chemical conversion of a derivatives of sarpagine into an identical one from ajmaline. Though the sequence of reactions employed was more laborious and probably required much more material than ours, i t led to compounds identical in every respect, which in addition permitted the assignment of the absolute stereochemistry, except (11) K g Dinh-Nguyen and R Ryhage, Acta Chem S c a n d , 13, 1032 (1959); J ReIearch Inst Calalysis, Hokkardo U n z s , 8 , 7 3 (1960). (12) M F Bsrtlett, R . Sklar and W I Taylor, J A m Chenz S o c , 8 2 , 3790 (1960)
198
253
28 I
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295
Fig. 2.-Partial mass spectra of O-methyldeoxydihydro: (a) undeuterated, ( b ) monodeuterated a t sarpagine (Ir) C17, (c) containing up t o six atoms of deuterium a t Cia, CIS and C ~ O
the steric relationship of C21 with regard to C18, as shown in VI.
Discussion of the Mass Spectra.-A detailed discussion of these mass spectra is now in order for two reasons. First, we have not yet presented any evidence that only indole alkaloids with such a carbon skeleton give this type of mass spectrum and, secondly, a wider knowledge about the fragmentation of such molecules may make i t possible to deduce the structure of an alkaloid with a closely related but not identical alicyclic carbon skeleton. The molecular weight is clearly indicated by the peak a t mass 310, which is due to the monisotopic species, while the peaks a t masses 311 and 312 are due to the molecules containing C13, N15, H2 and 0I8. Unless stated otherwise, the mass numbers used here and in the following discussion refer to the spectrum of V (Fig. la). The fragments of mass 295 and 281 have been already discussed and found to be due to the loss of one of the two methyl groups (C17 or CIS)and to loss of the ethyl group (CI,,,,). The strong peak a t mass 309 indicates the preferred loss of one hydrogen atom, which is in contrast to all other indole alkaloids that we have thus far examined. It implies that there is no other bond cleaved with particular ease. Loss of hydrogen from Ce or, less likely, from Czoseems to be the only possibility for the formation of an ion of mass 309 in which the positive
Vol. 83
K. BIEMANN
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charge would be well stabilized and where the resonance form would not involve a double bond on a bridge-head, The latter would be the case in an ion arising from the loss of hydrogen from C,, C g or C21. Removal of the tertiary hydrogen a t Cle would create a bridgehead carbonium ion which is also unfavorable.
tion of a doiible bond by loss of a hydrogen radical from C21 forms the fragment of mass 253 in step 4. 'l'he intense peak a t mass 198 arises by loss of butylene from the fragment of mass 254 yiu homolytic cleavage of the N\T~-C~I bond (step 5j and represents the molecular ion of rnethoxy-&carboline. As such it is in agreement with the high intensity of this peak indicating a very stable structure which also does not fragment further. The particle of mass 199 is the corresponding protonated form derived from mass 254, which is able to eliminate first a hydrogen atom from CI9(step G j and then a molecule of butadiene (step 7). The complex mechanism suggested may seem to be mere speculation. However, i t is proposed not only because. i t fulfills the requirements noted earlier but is in some phases in agreement with the known behavior of much simpler molecules. Strong support is derived also from the fact that the spectrum of the ajmaline derivative I V ? when obtained with bombarding electrons of low energy (9-10 eV.), indicates that the formation of the fragments of mass 182 and particularIy 238 (corresponding t o 198 and 254 in V) require the least energy for their formation from the molecular ion, while the peaks a t mass 183 and 237 (corresponding to 199 and 253 in V) disappear almost completely. This is in agreement with the suggestion that in the formation of mass 198 and 254 only neutral molecules are eliminated while steps 4 and G both involve the additional rupture of a bond with the formation J CHjO of a hydrogen radical. Finally, steps 3, 4 and h are \ supported by the presence of metastable peaks14 in the spectrum of IV: 294 +. 238, calcd. 192.7, found 193.5; 238 + 237, calcd. 236.0, found 236; 23s 182, calcd. 139.2, found 140. The lower intensity 199 of the spectrum of V, caused by the much lower I n the formation of the other important fragments (mass 198, 199, 253) more than one bond has vapor pressure of this substance, prohibited these to be broken. This has to occur in a manner which determinations for V. It is of interest to note that there are no peaks of leads to stable positive ions and avoids the accumulation of unpaired electrons since the over-all proc- appreciable intensity present which could be due ess must be an energetically favorable one to give to fragments representing parts of the alicyclic rise to rather intense peaks. Scheme I illustrates a system of the molecule after loss of the aromatic plausible mechanism fulfilling these requirements moiety. Such peaks are quite abundant in the specand is also in agreement with the spectra of the two tra of other indole alkaloids, such as ibogaine and deuterated species of V (Fig. 2). The latter indi- its congeners and also dihvdroindoles of the aspidocate that C a is lost in the formation of the frag- spermine type which will be discussed in following ments of mass 253, 199 and 198 and also that the papers of this series. I n the sarpagine skeleton the two last-mentioned fragments do not contain CIS, highly stabilized P-carboline system is already preformed and we believe it is for this reason that this C19 and (220. The initial removal of one electron, to give the ring system is present in all the more important molecular ion, is followed by cleavage (wiggled fragments observed, arrow) of the C&la bond in such a mamer as to In that respect there is a certain analogy to be retain one electron on while the positive charge noted between the behavior of these molecules and a t Cs is particularly stabilized, not only by the free their thernial dcgradation. Both ibogainelj and electron pair of Nq, but also ot N1 by participation aspidospermine'6 yield in considerable amounts, on of the n-electron system of the pyrrole ring (step heating with potassium hydroxide, zinc dust or 1). A shift of electrons (step 2) simultaneously selenium, pyridine derivatives while sarpagine does breaks a single bond, forms a double bond, and not. converts a primary radical t o a secondary one. (14) For the origin of metastable peaks see J. A. Ilipple and E. U. Concerted rearrangement of a hydrogen and elim- Condon, P h y s . R e t . , 68,54 (1945). Its significance for the interpretaination of butylene (step 3) gives rise to an ion- tion has been discussed recently (K. Biemann, J. Seib! and F. G a m , radical of mass 254. This process is analogous to 1.A m . Cheir. Soc., 83, 3795 (1861)). (1.5) R. Goutarel, M.-M. Janot, F. Mathys and V. Prelog, C o m p f . the behavior of simple, aliphatic olefins, in which 237, 1718 (1953); R. Goutarel, F. Percheron, J. Wohlfahrt and such eliminations have been observed.la Forma- rend., ?vl.-h.I. Janot, A n n . phorm. franc., 15, 353 (1957).
vh-7f-l
(13) P. W. McLafferty, A n d . C h a m . 31, 2072 (1959).
-
(1G):B.
Witkop, J. A m . Chem. Soc., 7 0 , 3712 (1948)
Dec. 5, 1961
MASSSPECTROMETRY OF IBOGA ALKALOIDS E~perimental'~
0-Methylsarpagine (Ib).-Trimethylanilinium iodide (0.40 9.) in 10 ml. of hot ethanol was stirred with excess silver oxide for a few minutes, centrifuged and the supernatant evaporated. T o the sirupy rcsidue was added 200 mg. of sarpagine ( I a ) and 0.45 ml. of dimethylaniline. After heating this mixture t o 165" for 1.5 hours, it was poured into dilute acctic acid and extracted with ether. The aqueous layer was then made alkaline with sodium hydroxide and extracted with dichloromethane-ether. The residue of the organic layer was chromatographed on alumina (neutral, 11); chloroform eluted 49 mg.; after recrystallization from ethanol-water 32.6 mg. of 0-methylsarpagine, m.p. 201-203° (lit.S202-202.5"), was obtained. 0-Methy1deoxysarpagine.-0-Methylsarpagine (29.3 mg.) was tosylateds using 25 mg. of toluolsulfochloride in 0.5 ml. of pyridine. The crude amorphous product (IC) was divided into several parts and reduced: ( a ) With lithium aluminum hydride.-Half of the above product ICwas reduced with 32 mg. of LiA1H4 in 2 ml. of tetrahydrofuran yielding 7.8 mg. of 0-methyldeoxysarpagine, m .p. 248250" (lit.* 248-250"), mol. wt. 308 (by mass spectrometry). ( b ) With lithium aluminum deuteride.-One-quarter of the crude tosylate ICwas reduced in the same way, using 18 mg. of LiAlI),. The crude product 5.1 mg. was recrystallized, (17) The spectra were determiued with a CEC 21-103C mass spectrometer, equipped with heated inlet system, operated a t 140°. Samples of 60-250 micrograms were employed. Melting points were determined on a Kofler micro-hot-stage.
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giving 2.0 nig. of 0-methyldeoxysarpagine-dl, m.p. 247249', mol. wt. 309 (by massspectrornetry). 0-Methyldeoxydihydrosarpagine (V).-Two milligrams of 0-methyldeoxysarpagine were hydrogenated in ethanol, using platinum as a catalyst. After 20 min. the product was isolated and sublimed a t 0.1 mm. and 200" (bath); m.p. 237-240'. The mol. wt. of 310 (by mass spectrometry, see Fig. l a ) indicated the formation of a dihydro derivative. 0-Methyldeoxydihydrosarpagine-dl was prepared in the same manner by catalytic hydrogenation of O-methyldeoxysarpagine-dl; mol. wt. 311 (by mass spectrometry, see Fig. 2b). O-Methyldeoxydihydrosarpagine-do-ds .-When O-methyldeoxysarpagine (1.5 mg.) was catalytically deuterated, using DrMeOD-Pt and worked u p as described for the hydrogenations above including repested evaporation with ethanol, the mass spectrum indicated a mixture of species containing up t o seven deuterium atoms per molecule, the major component having a mol. wt. of 313 (-trideuterio derivative). Equilibration of this "mixture" with H2-EtOH-Pt gave rise t o the apparent loss of one deuterium atom. The mass spectrum of this material is shown in Fig. 2c.
Acknowledgments.-The author is greatly indebted to Prof. R. B. IVoodward for a sample of compound IV, to Dr. A. Hofmann for sarpagine, and to the National Science Foundation for financial support (Grant G505l).
[CONTRIBUTION FROivl THE DEPARTMENT OF CHEMISTRY, MASSACHUSETTS INSTITUTE OF TECHNOLOGY, C A Y B N D G E 39, MASS.]
Application of Mass Spectrometry to Structure Problems.
V.I
Iboga Alkaloids2
BY I
